The present invention relates generally to analog gyroscopes, and more particularly to an analog fiber optic gyroscope system which provides an extended linear dynamic range.
Gyroscopes are utilized in a variety of different applications including as rate sensors, tachometers for generator speed control, and inertial navigation and non-magnetic compasses. In its most common application, the gyroscope is disposed in a gimbal mounting platform and used for stabilizing a body about one or more mutually perpendicular axes. The device so mounted may be used to stabilize a space platform with respect to the system of fixed stars either as a visual aid in navigation or to supply information for actuating automatic piloting or navigating mechanisms.
It has been known for some time that the Sagnac interferometer can be used to detect the rotation rate of any rotating frame, and thus can be used as a gyroscope. The Sagnac interferometer is based on the existence of a measurable phase shifting effect caused by angular motion upon the transmission and subsequent interference of counter propagating electromagnetic waves in a light circuit loop path disposed in the plane of the angular motion. By way of example, if an optical fiber coil is disposed on a platform, with the axis of the coil perpendicular to the platform plane, and the platform is at rest, then co-rotating and counter-rotating optical beams propagating in the fiber optic coil suffer exactly the same time delay in propagating in opposite directions through the coil. Accordingly, when these beams meet, they interfere to provide a certain light amplitude output. However, when the platform is rotating, there is a slight difference between the propagation times through the coil of on the order of 10.sup.-3 ps due to the effects of relativity. This time delay will cause a change in interference which will change the light intensity. The change in the light intensity is proportional to the rotation rate.
Fiber optic gyroscopes based on the foregoing Sagnac effect are of substantial current interest for the previously noted applications, but must satisfy stringent specifications, including linearity of output. However, since the Sagnac interferometer, in its simple form, is an analog device, it is typically limited to a linear range of approximately 40 dB, whereas 60 dB is often required. One method of obtaining the 60 dB linear range is to utilize a phase nulling scheme as proposed by Cahill and Udd, Applied Optics 19, 3054 (1980). This reported design basically shifts the frequency of the counterpropagating beams in order to null the Sagnac effect, and then measures frequency shift. The problem with this approach is that it requires bulk optic frequency shifters. Thus, the design has a high power requirement to operate these frequency shifters and, it cannot easily be implemented in an all fiber format.